a) Field of the Invention
The present invention relates to a laser beam emission apparatus for use as a universal optical pickup laser light source capable of recording/playback even from optical disks recorded in different recording modes.
b) Description of the Related Art
Optical disks are now finding use not only as CDS (compact disks) but also as DVDs (digital video disks). This trend has led to studies on universal optical pickups that can be used for playback in both CD and DVD systems.
Because the recording density is much higher in DVDs than it is in CDS, a smaller-diameter light spot must be formed on the surface of the disk for playback of this high-density recorded data. In general, the diameter of the light spot formed on the disk is proportional to the wavelength of the laser beam being used. Therefore, whereas laser light sources in CD systems have a wavelength of 780 nm, DVD systems require lasers with shorter wavelengths of 635 or 650 nm. Specifically, 780-nm AlGaAs semiconductor lasers are used for CDS, and 635 or 650 nm AlGaInP semiconductor lasers are used for DVDs.
Therefore, a universal optical pickup that uses a 635 nm or 650 nm laser light source for DVD playback, should also be capable of playing-back the lower density CD recordings. In rewriteable CD-Rs, however, the 635/650 nm wavelength band is in the absorption band of the medium. Thus light in this wavelength band has extremely low reflectivity in CD-R recording media, making it unsuitable for playback of CD-R data.
This led to the development of an optical pickup capable of recording/playback of both DVD and CD-R disks, in a configuration that uses two lasers, each oscillating at a different wavelength, the appropriate laser being switch-selected according to the type of disk being used. Such a system was disclosed in Japanese Patent Kokai (laid-open application) No. 8-55363. The optical system associated with the optical pickup in this system is configured so that the laser beam from either of two semiconductor lasers mounted in different locations can be routed through a polarized wavelength filter into a common light path, thus allowing the proper semiconductor laser for the optical disk being played-back to be switch-selected. It would be very difficult, however, to miniaturize such a system.